1. Field of the Invention
The invention relates to a device for regulating the delivery of rotary compressors, more particularly screw-type compressors with oil injection, comprising a regulating valve for varying the cross-sectional area of the suction pipe and a non-return valve tightly closing the suction pipe, the two valves being disposed one behind the other in the flow direction in a casing incorporated in the suction pipe, the regulating valve being constructed as a flap valve having a flap mechanically adjustable by a drive device around an axis of rotation extending diametrically through the flap.
2. Description of the Prior Art
It is known, when operating rotary compressors, to use a suction regulating flap incorporated in the suction pipe to vary the cross-sectional area of the suction pipe in order to adapt the delivery to requirements. The suction pipe must also be tightly closed after the compressor has been switched off, to prevent the compressed medium flowing back through the suction pipe. This is particularly necessary in screw-type compressors, into which oil is injected for lubrication or cooling.
It is known to construct the suction regulating flap so that it also can tightly close the suction pipe. This double purpose, however, is relatively expensive to service, because during operation the suction regulating flap for regulating the delivery is actuated by the drive device, which must be constructed so that it does not interfere with the suction regulating flap after the compressor has been switched off and when the suction pipe is rapidly closed. In addition, suction regulating flaps of the kind in question are difficult to make completely sealing-tight.
To obviate these disadvantages, it is known for a separate non-return valve of known construction to be incorporated in the suction pipe in addition to the suction-regulating flap. The components for closing the non-return valve are loaded by a closing spring, the force of which has to be overcome when opening the valve. The closure components therefore often have a considerable flow resistance, causing additional pressure losses in the suction pipe and consequently reducing the delivery of the compressor, precisely when the full delivery of the compressor is required and no excess capacity is at hand.
U.S. Pat. No. 4,406,588 discloses a suction control device in which the closure member for closing the suction pipe is biased in the closing direction by a spring and in the opening direction by the pressure produced by the compressor. When the compressor starts up, the resulting pressure overcomes the force of the spring and opens the suction pipe. The flow cross-section for regulating the delivery is adjusted by a control piston actuated by a control pressure, e.g. the main pressure, and loading the closure member in the same direction as the force of the spring, so as to overcome the operating pressure on the other side of the closure member. This embodiment eliminates a separate non-return valve, but the known suction control device is of complicated construction, resulting in high prime costs and faults in operation. As before, the spring of the valve which closes the suction pipe causes pressure losses and thus reduces the maximum possible delivery of the compressor.